Unlike other energy sources for power generation such as solar and wind, geothermal energy is stored deep under the surface. Exploration activity and drilling are required to find and confirm the presence and magnitude of the resource before the feasibility of the project can be assessed. This high upfront cost in the early stage of the project while the uncertainty is still very high makes investor reluctant to invest in the geothermal exploration activity. One alternative to reduce the resource risk during exploration is by using less expensive drilling method such as slim hole drilling instead of using standard or big hole drilling. Slim hole drilling was used for early geothermal exploration in Indonesia since 1993, but the recent trend in Indonesia shows that most geothermal developers prefer to use standard or big hole instead for exploration activity. This may cause a huge loss for the developer if they are unable to confirm the geothermal resource during exploration drilling, and currently Government of Indonesia considers using slim hole drilling for reducing the resource risk in the exploration. The objectives of this paper are to give an overview of slim hole drilling and how it differs from standard or big hole, both from a technical and economic aspect. The advantages, limitation, and challenges of slim hole drilling application for geothermal exploration in Indonesia are also discussed. A case study from recent slim hole drilling in East Java is used for an example of current slim hole utilization for geothermal exploration. The lessons learned from the case study then can be used to improve exploration and drilling planning in the future.

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